In a solid-phase support system disclosed in patent application Ser. No. 627,941, filed Nov. 3, 1975, a sample is sorbed onto a reaction support surface (e.g., disc) carried by a diagnostic reagent holder. Antigen or antibody in the sample is labelled as with a fluorogen for quantitative reading of the surface by a fluorometer through a window in a viewing housing which precisely positions the surface. The disc and holder are illustrated in a vertical position.